1. Field of the Invention
This invention relates to prostheses for replacing structures of the human spine and more particularly to prostheses for replacing a facet joint.
2. Brief Description of the Prior Art
Lower back pain is a very common disorder and is responsible for extensive morbidity and lost time at work. The prevalence rate of low back pain is very high, affecting approximately 80% of the general population at some time. Although most patients experience the painful symptoms only occasionally and recover fully, approximately 10% of these patients experience chronic and disabling low back pain in spite of various medical treatments.
The most common cause of chronic disabling low back pain is degenerative disk disease (DDD). Another problem associated with low back pain, which often accompanies DDD is degeneration of the facet joints between vertebrae.
Anatomy and Biomechanics of the Facet Joints:
The functional unit of the spinal column is a spinal motion segment that is made of a three-joint complex, a disc and two facet joints. The facet joint is a synovial joint with the joint surfaces covered by hyaline cartilage like other diarthrodial joints. The orientation of the facet joints in the lumbosacral spine is symmetrical on both sides in almost all individuals, but it is occasionally found to be asymmetrical. Facet asymmetry has been reported in the literature to cause disc degeneration. The facet joint is generally oriented obliquely in sagittal and coronal planes. The orientation of the facet joints is significantly different at different levels within the spine, i.e., the cervical, thoracic, thoraco-lumbar, and lumbo-sacral regions of the spine. Within the lumbar or lumbo-sacral spine the size, shape, orientation, and angle of the facet joints have a wide range of variation among the motion segment ins different levels within an individual. Such variations are even greater among different individuals.
Although the disc is the principal structure performing the functions of weight bearing, stabilization, and motion in a spinal motion segment, the facet joints also contribute significantly to these functions.
Weight bearing function: The facet joints are responsible for approximately 20% of the weight bearing function of a spinal motion segment at the neutral erect posture. The weight bearing function of facet joints decreases on flexed posture and increases on extension (up to 40%). Biomechanical studies have shown that the facet joint capsule primarily takes up the weight bearing function of the facet joints in the axial direction. Such studies include: Al-Bohy A A, Yang K H, King A I: Experimental verification of facet load transmission by direct measurement of facet lamina contact pressure. J Biomech. 22: 931-941, 1989, and Yang K H, King A I: Mechanism of facet load transmission as a hypothesis for low back pain. Spine 9: 557-565, 1984.
Motion: The facet joints, two opposing articular processes, glide during flexion-extension, rotate during torsion, and toggle during lateral bending of the spinal motion segment. The motion at the facet joints is not a mere passive motion, but a positively guided motion by the shape, orientation, and angle of the facets and by the joint capsule. In a normal spinal motion segment, the facet joints become “locked” during extension, thus allowing less rotational and gliding motion to promote weight-bearing stability. In flexion, the facet joints are less engaged, which allows for freer motion.
Stability of a spinal motion segment: The disc is the principal stabilizer for an intact spinal motion segment. However, the facet joints are important stabilizing structures for torsion and shear stability in the spinal motion segment. The facet joints are responsible for approximately 45-50% of the torsional stability, and the disc is responsible for approximately 50-55% of the torsional stability. Torsional stability of the spinal motion segment is discussed in: Farfan H F, Cossette J W, Robertson G H, et al: The effects of torsion on the lumbar intervertebral joints: The role of torsion in the production of disc degeneration. J Bone and Joint Surg. 52A: 468-497, 1970. When the disc degenerates and loses its stabilizing function, the facet joints become an even more important stabilizer. Facet joints are important structures for protecting the disc by limiting excessive torsional and shear motion.
Structural changes of the facet joint are usually seen in the late stage of degeneration of a spinal motion segment. Isolated facet joint degeneration in the absence of disc degeneration is very rare. The pathology of the degenerated facet joint is very similar to that observed in other weight bearing synovial joints, and includes synovial and capsular hypertrophy, joint effusion/cyst, bony hypertrophy, and/or joint subluxation. Hypertrophy of the facet joint capsule, synovium or bone may cause spinal stenosis. Decompression with or without spinal fusion is sometimes indicated for patients with chronic disabling low back pain or stenotic symptoms caused by severe degenerative changes of the facet joints that are not responding to non-operative treatments.
Decompression for spinal stenosis provides successful relief of symptoms, but the recurrence rate of symptoms is very high (40-50% in 5 years). Spinal fusion often provides good results but has adverse effects. Novel ideas of replacement of a painful and dysfunctional disc with an artificial disc prosthesis have been evolved recently. Some disc prosthesis designs have been used clinically in recent years. The artificial disc prosthesis is best indicated for patients with painful DDD with no or little facet joint degeneration. In advanced degenerative changes of a spinal motion segment, all three joints (the disc and facet joints) are affected, and replacement of the disc alone will not provide satisfactory results. Replacement of all three joints (a disc and two facet joints) may be required for satisfactory results with restoration of the motion segment function.
The requirements for a successful artificial facet joints prosthesis are: 1) It should provide an adequate range of motion for flexion-extension glide, rotation, and toggle during lateral bending of the disc. 2) It should provide stability, especially for torsional and shear motion. 3) It should provide a weight-bearing function of 20-30% of the physiological load. 4) The prosthetic components should have adequate fixation to the bone to overcome a very repetitive and a high bending moment and shear forces especially on the superior articular process. 5) It should provide a positive guidance of motion, especially for rotation and for flexion and extension. 6) It should be user-friendly, by overcoming the problem of a wide range of variability of size, shape, angle, and orientation. A prosthesis that lacks any one of these features will be prone to malfunction or become loose and thereby place untoward stress on the disc and the adjacent levels.
A number of attempts have been made to provide a satisfactory prosthesis for replacing the human facet joints.
U.S. Pat. No. 5,571,191, (and Reissue Pat. No. RE36,758) to William R. Fitz, entitled “Artificial Facet Joint” discloses a prosthesis having a superior component of conical or pyramidal shape that is fixed to the distal portion of the inferior articular process, e.g., with a bone screw or the like. The inferior component of the prosthesis is also roughly conical or pyramidal with one side somewhat elongated posteriorly and medially. It is distally fastened to the superior articular process. This prosthesis functions primarily as a surface interposition on the facet joints by capping the inferior and superior articular processes without altering the bony anatomy. The caps are fixed to the underlying bony articular processes by screws, and may have a porous coating on their interior surfaces to promote ingrowth of bone.
However, because of the very wide variation among individual patients in angle, orientation, size, and shape of the facets, a single set of conical or pyramidal caps will not be capable of matching all bone structures found in various individuals. In order to match all the naturally occurring variations, an almost unlimited number of sets of caps having different sizes, shapes, angles and orientations would be required. Another problem that may be encountered with this design is related to the stability of the implanted prosthetic device. Simple fixation of the caps to the underlying articular processes may not withstand repetitive torsional, bending and shear forces. The generally conical internal shape of the superior component may allow motion at the interface between the bone of the inferior articular process and the prosthesis. Furthermore, the inferior component, fastened to the superior articular process, may also not withstand a very high bending moment and shear force. Finally, the articulating surfaces between the inferior and superior components may not provide stability during torsional motion, especially during compression-torsion motion. Also this prosthesis may have difficulty in providing a substantial weight bearing function in the axial direction.
U.S. Patent application No. 2002/0123806 to Mark A. Reiley, (now U.S. Pat. No. 6,610,091) entitled “Facet Arthroplasty Devices and Methods”, discloses a universal facet prosthesis having a concentric ball (inferior facet) and saucer or shallow socket (superior facet). It is designed for replacement of the lamina, superior articular processes and inferior articular processes after resection of those structures, unilaterally or bilaterally. The parts of the prosthesis are fixed into the vertebral bone through the pedicle with a screw or a peg. Alternatively, they may be fixed to the spinous process. Unilateral or bilateral variations are disclosed.
In this design, the concentric ball and shallow socket (or saucer) may have difficulty in meeting the above requirements, especially the requirement to provide positive torsional stability and shear stability during flexion of the motion segment. The inability of the vertebral motion segment structure to control the torsion during compression-flexion was found, in a biomechanical study by Farfan, (Farfan H F, Cossette J W, Robertson G H, et al: The effects of torsion on the lumbar intervertebral joints: The role of torsion in the production of disc degeneration, J. Bone and Joint Surg. 52A: 468-497, 1970), to be the most significant cause of injury to the disk. In the normal spinal motion segment, the rotary motion has its center near the posterior vertebral body (in front of the facet joints), and is accompanied by lateral-medial displacement of the inferior facets with respect to the superior facets on both sides. However, a concentric ball and socket or saucer design will be expected to have difficulty in reproducing the natural action of the facet joints during spinal rotation. Furthermore, single-point fixation into the vertebral body through the pedicle may not provide satisfactory fixation of the device so that it is capable of withstanding the repetitive stress involving the large bending moment and shear force acting upon the superior articular prosthesis during compression-flexion and compression-torsion.
U.S. Pat. No. 6,132,464, to Jean-Raymond Martin, entitled “Vertebral Joint Facets Prosthesis” discloses a prosthesis comprising two synthetic sliding surfaces in contact, one for the superior face and one for the inferior facet. The underlying bony structures are undisturbed, and these sliding surfaces cover the superior and inferior facets. The device is fixed in position by screw fixation into the pedicle, or by other means such as hooks, claws, or a clamping collar around the transverse process, or by a support plate or fixation to the spinous process.
However, because of the wide natural variation in the anatomy of the facet joints, it is expected to be difficult to provide a proper fitting or contouring of the prosthesis to the underlying bony structure. Because of the great anatomical variation among individuals, and the variation among the different motion segments within an individual, it may require a great many different prostheses having varied sizes, shapes, and orientations. Furthermore, in this prosthesis, the structure that connects the two sliding surfaces of the facets is located anterior-lateral to the pars interarticularis of the lamina. However, the exiting dorsal nerve root and the post-ganglion nerve also pass through this region. Consequently, crowding of the neural structures may be a problem.
U.S. Pat. No. 6,132,464, to T. Wade Fallin, entitled “Prosthesis for replacement of a Posterior Element of a Vertebra”, discloses a prosthesis designed to replace all of the posterior spinal structures after resection of the spinous process, bilateral facet joints, and lamina. The basic unit comprises a prosthetic lamina with concave/convex-shaped “blades” for articular facets. Alternate embodiments include structure, in addition to the basic unit, to replace the spinous process, transverse processes, and/or the pedicle. Fixation of the device is accomplished by screws fixed in the pedicles.
However, because the natural facets are replaced by convex/concave sliding blades, the device may lack positive motion guidance or stability. Furthermore, a question may be raised as to whether fixation to the pedicles provides a sufficiently secure fixation for a permanent prosthesis.
In summary, certain problems are apparent in most of the previous designs for facet joint prostheses.
1) The contact surface area, the quantity and direction of force transmitted from the inferior articular process of the cephalad vertebra to the superior articular process of the caudad vertebra changes constantly during the normal range of motion of a spinal motion segment. Any prosthetic device that has fixed-angle contact surface areas, such as a ball-and-socket or ball-and-saucer joint may not be able to provide a wide variable arc of motion, stability and weight-bearing function.
2) The facet joint is a very important joint for providing stability against anterior shear during flexion, when the bending moment acting on the superior articular facet is relatively large. Permanent fixation of the prosthetic superior articular process or surface to the underlying bone is critical. The bony mass of the superior articular process is too small for adequate mechanical fixation by screws, pins, or pegs, or by capping to withstand such a very large bending moment (½ of the body weight×8-16 inches bending moment during flexion).
3) A prosthesis device for the inferior articular process that is fixed thereto through a structure having a substantially circular or oval cross-section may well be prone to loosen under the stress of rotational force.
Accordingly, a need has continued to exist for a facet joint prosthesis that is not subject to the deficiencies of the hitherto available prostheses.